WO2009054819A1

WO2009054819A1 - Production of tungsten and tungsten alloys from tungsten bearing compounds by electrochemical methods

Info

Publication number: WO2009054819A1
Application number: PCT/TR2008/000101
Authority: WO
Inventors: Ishak Karakaya; Metehan Erdogan
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-10-22
Filing date: 2008-08-15
Publication date: 2009-04-30
Anticipated expiration: 2010-04-22
Also published as: CA2703400C; TR200707197A1; CA2703400A1; RU2463387C2

Abstract

This invention relates to a method for producing tungsten and/or tungsten alloys from tungsten bearing compounds by electro reduction in molten salt solutions. The process for carrying out the invention can be expanded to include other metals, which show similar behavior with tungsten. This invention relates to a method for excluding non-metallic compounds (like X) from metallic compounds (like M1M2X) by electrolysis inside melted salt or salt solutions.

Description

DESCRIPTION

PRODUCTION OF TUNGSTEN AND TUNGSTEN ALLOYS FROM TUNGSTEN BEARING

COMPOUNDS BY ELECTROCHEMICAL METHODS

Related Field of The Invention:

This invention relates to a method for producing tungsten and/or tungsten alloys from tungsten bearing compounds by electroreduction in molten salt solutions. The process for carrying out the invention can be expanded to include other metals, which show similar behavior with tungsten.

Background to the invention:

Metal production by molten salt electrolysis is used for more than a century. Metals can not be produced by gas-based pyro-reduction, metallothermic reduction, hydrometallurgical methods or aqueous electrochemical techniques if their compounds are very stable. In these cases, where production by other routes is not feasible due to thermodynamic, kinetic or economical reasons, the choice is limited to electrowon the metal by the electrolysis of various molten salts. However fused salt electrolysis is satisfactory only for low melting point metals which are deposited in the liquid state. High melting point metals can be electrowon in the solid state because the reduction temperature is less than the melting point; but this results in dendritic deposition which is prone to oxidation.

The classical molten salt electrolysis involves dissolution of the starting material, containing the metal to be electrowon, in the electrolyte. A subsequent electrowinning of the metal from the solution takes place by direct current reduction. Many metals exist as oxides or oxygen bearing metal compounds in nature. Therefore, there had been some attempts to produce metals from their oxides by molten salt electrolysis method. In these work, metal oxides were dissolved in fluoride melts because fluorides are regarded to be better at dissolving oxides than chlorides. However, most of the time these studies failed to develop a commercial large scale production because fluorides usually melt at higher temperatures and they are far more corrosive. Until quite recently, the only molten salt process involving electrochemical reduction of oxides industrially was the Hall-Heroult process for the electrowinning of aluminum from alumina. In 1997, a novel process has been reported for the production of metals from their oxides by electrolysis in fused salts. The process is called as FFC (Fray-Farthing-Chen) Cambridge process and it is claimed to be more suitable for electroreduction of the high- melting transition metal oxides and actinides. FFC Cambridge Process is a high temperature electrolysis process in molten salts to obtain metals or alloys from the starting material as oxide, sulphide, carbide or nitride. By this process, metals or metal alloys can be produced without dissolution of the starting material in the electrolyte and continuous electrolysis of the molten salts. The patent on the process (WO 99/64638) claims that Ti, Si, Ge, Zr, Hf, Sm, U, Al, Mg, Nd, Mo, Cr, Nb and any alloys thereof could be produced by FFC Cambridge process.

Today's only technically important method for tungsten powder production is the hydrogen reduction of tungsten oxides at temperatures between 600 - 1100 ^°C in a streaming hydrogen atmosphere. The overall reduction reaction can be given as:

WO₃ (S) + 3H₂ (g) = W (s) + 3H₂O (g) It has been estimated that, 70% of total tungsten world reserves is scheelite (CaWO₄) and 30% is Wolframite ((Fe, Mn)WO₄). Production of tungsten oxides from these minerals is a complicated and time consuming procedure. Furthermore, Gibbs free energy change of the above reaction is not a large negative value; which means there is considerably low driving force for the process at the temperatures of reduction. In addition, since the above reaction is endothermic, in order to attain and preserve the high temperatures required for reduction, continuous external heat supply is necessary.

Extensive research in recent years has failed to develop any essentially new method of large-scale tungsten production. In those studies which aim to produce tungsten by electrochemical methods, the most important problem is the large particle size of the electrowon tungsten in dendritic deposition. This leads to a relatively porous product when consolidated by subsequent classical pressing and sintering techniques. There are some patents about production of tungsten via electrochemical methods. However, in all of these patents, the processes described were classical molten salt electrolysis techniques, in which the starting materials were first dissolved in the electrolyte and subsequently electrowon from the solution by direct current reduction. Brief Description of the Invention:

According to the present invention, a method for removing a substance (X) from a metal compound (M¹M²X) by electrolysis in a molten salt or in molten salt solutions. According to the one embodiment of the invention, M¹M²X cathode is a conductor.

Alternatively, M¹M²X may be an insulator in contact with a conductor.

In a preferred embodiment, M¹M²X is any compound having scheelite stochiometry (M¹M²O₄).

In a preferred embodiment, M¹ is any of W, Mo or any alloy thereof. In a further preferred embodiment, M² is any of Ca, Fe, Ba, Mn, Pb, Cd, Sr or any alloy thereof.

In a further preferred embodiment, X is any of O, S, C, or N.

In a still further preferred embodiment, the electrolyte is a molten chloride salt or chloride salt solutions. In the method of invention, electrolysis preferably occurs with a potential below the decomposition potential of the electrolyte.

This invention is an alternative tungsten production technique. The process for carrying out the invention is more direct and cheaper than the usual production technique. Furthermore, the invention can be expanded to include other metals which have similar characteristics with tungsten.

Detailed Description of the Invention:

It is not possible to produce tungsten from tungsten oxide without loss, by the FFC Cambridge Process described above. The reason is that, at the temperatures of reduction, tungsten oxide reacts with calcium chloride by liberating volatile tungsten oxychloride.

2WO₃ (s) + CaCI₂ (I) = CaWO₄ (s) + WO₂CI₂ (g)

However, in accordance with this invention, it has been observed that if CaWO₄ (one of the products of the above reaction) was formed before the electrolysis, and used as the startinα material instead of WO?: the above reaction could be inhibited. Furthermore. was stated earlier, nearly 70% of total tungsten world reserves are scheelite (CaWO₄). Ongoing experiments demonstrated that the process described here, has the potential to be directly applied to the scheelite concentrate. Therefore, present invention could remove most of the intermediate steps of the current tungsten production technique. In the present invention, the cathode of the cell is tungsten (or any metal which has similar characteristics with tungsten) compounds. In order to act as cathode, these compounds need to be either conductors, or used in contact with a conductor. As the anode material graphite, carbon or an inert metal could be used.

This invention is based on the removal of non-metallic substances from metal compounds in molten salts, without continuous electrolysis of the electrolyte. In order to ease the diffusion of the non-metallic substances, it is advantageous to use the cathode materials porous.

In this invention it is important that the starting materials should not have serious solubility in the electrolyte. This situation may impose a restriction on the temperature range over which electrolysis can be done. As the electrolyte, chloride salts are chosen. These salts can be used alone or as salt solutions in order to control the temperature and solubility of the cathode material. In order to have more freedom about voltage selection, it is an important advantage to use electrolytes with decomposition potentials as high as possible.

The following example is given to illustrate the process:

Example

Synthetically produced calcium tungstate (CaWO₄) powder was pressed in the form of a pellet of weight 2.5 grams, diameter 1.5 cm and height 0.3 cm. Then this porous CaWO₄ pellet, forming the cathode of the cell, was placed into a spoon (made up of stainless steel), which was attached to the end of a stainless steel wire current collector. A graphite rod was used as the anode material and the electrolyte was CaCI₂-NaCI salt mixture at eutectic composition. At 600^°C the electrodes were immersed into the electrolyte, and a potential difference of 2.5 volts was applied between the electrodes for 500 minutes. After the electrolysis experiment, electrodes were removed from the molten salt solution and left to cool. Following the cooling period, the cathode sample was washed filtered and dried. X-ray diffraction analysis of the sample taken from the cathode revealed significant concentrations Present invention is an alternative tungsten production technique. It is easier and cheaper than the current production technique (reduction of tungsten oxides by hydrogen gas). It has the potential to be directly applied to tungsten concentrates. Feasibility of electroreduction of tungsten concentrates may remove many intermediate steps in today's tungsten production method. Finally the process given in present can be expanded to include other metals which show similar behavior with tungsten.

Claims

1. A method for removing a substance (X) from a metal compound (M¹M²X) by electrolysis in a fused salt or a fused salt solution.

2. A method as claimed in claim 1, wherein M¹M²X cathode is a conductor.

3. A method as claimed in claim 1, wherein M¹M²X cathode is an insulator and used in contact with a conductor.

4. A method as claimed in any preceding claim, wherein M¹ is W or Mo or any alloy thereof.

5. A method as claimed in any preceding claim, wherein M² is Ca, Fe, Ba, Mn, Pb, Cd, Sr or any alloy thereof.

6. A method as claimed in any preceding claim, wherein X is O, S, C, or N.

7. A method as claimed in any preceding claim, wherein chloride salts (alone or as salt mixtures) are used as the electrolyte material.

8. A method as claimed in any preceding claim, wherein the applied potential is lower than the decomposition potential of the electrolyte.

9. A method as claimed in any preceding claim, wherein M¹M²X is in the form of a porous pellet or powder.